Therapists are NOT medical professionals. There is research, but as we've seen in recent years, most of it is crap - see articles on p-values and meta analysis. The do produce some practitioners who produce better average results than zero intervention. They have also documented (if you believe their research) that some people are made worse by therapists. They seem to have a selection bias in their results too, anyone who dropped out is not included in results.
It can help. I've seen it help people. But there is no overthinking the selection of a therapist, they are not all the same by a long shot. Nor is their training and preferred modality of "treatment" the same - in fact some are contraindicated for certain conditions by some.
Therapy works by having another person serve as a kind of mirror. Just like ordinary mirrors allow you to recognise yourself visually, human mirrors allow you to see yourself "emotionally" (in broad terms).
We are mostly just awash with emotion and experience -- all of us -- and we only become aware that we are experiencing some emotion (etc.) when we encounter another person who reacts to it. On a day alone we might be very frustrated but we only realise when our frustration causes another to recoil. It is this recoil that provides the "reflection" of what we are feeling that we can see.
It is very hard to be aware of one's emotional state, and the capacity is trained and not particularly reliable in healthy people. But present enough to stabilize emotions.
A therapist is only a person who serves as a good mirror for your wife. A person who is able to recoil at the things a healthy parent ought recoil at, a person who is able to be warm and accepting at the things a parent ought be accepting of.
A therapist that hides behind a chair is useless. You're after someone around whom your wife isnt on-edge, and offers observations and emotive-physical reactions that your wife recognises and trusts.
In a sense "that is it". It doesnt matter what superstitons the therapist uses to arrive at their questions and suggestions. By being experienced and empathetic -- and "parental" -- the right person need have no particular theoretical background.
It's hard to say how easy it is to distinguish the good from the bad here. I guess you need to trust your intuition about how your wife feels in the situation. You need to have seen a few yourself and be aware that some people simply make you profoundly comfortable because of who they are -- and it is these you are looking for.
I am myself incredibly set on-edge by anyone i sense faking any emotion, so I cannot even be near therapists who smile too much (etc.). Fortunately, you do not need to "put up" with being in any sense on-edge. Find one that fits.
There is no science of human emotional instinct (yet). Your wife's issues are extremely unique to her, in the sense that they are connected to her particular experiences and ways of thinking. What is needed is an opening up about these, to create a safe environment in which your wife feels able to recognise her self, and to relearn an emotionality (, identity, self-awareness, understanding of consequences, appreciation of the future, understanding that relationships remain constant when people go out of view...) and other things besides.
PS.
As far as criteria go, I believe years of experience is the main one. While the mechanism of change, I believe, is the interaction between the therapist and patient -- more experienced therapists will be able to spot more subtle problems. Newer ones will be easy for a person with BPD to fool.
People with BPD are often, essentially, in a constant state of acting where they are fooling themselves with superstitions about who they are. The therapist needs to be alert to these superstitions being employed as explanations in answers to their questions. Ie., they need to be alert to your wife lying (but a lie she is not herself aware of).
By way of example, I observed a therapist once say to a patient that he wouldnt tolerate anyone being different than he was. He made this observation based on the patient wishing to be "understood". The connection was: the patient was misdescribed people "misunderstanding" him to actually their disagreeing with him.. and he couldn't tolerate disagreement.
The therapist made this suggestion base on their experience with this not uncommon issue (anger and hostility may often be a product of a fear of difference).